Account for the case in which Qt tools are not in PATH

Account for the case in which Qt tools are not in PATH

If the Qt directory is specified by --with-qt-dir, the full path
of the tools is retained. Otherwise, if qtchooser is detected,
the generic names are attempted with proper arguments to select
the desired Qt version. Otherwise, the generic names with and without
proper suffixes (either -qt5 or -qt4) are checked in the PATH.

This means that --with-qt-dir=qtdir should be used only if the tools
are not in the PATH, or qtdir/bin/{moc,rcc,uic} are the right versions
for the desired Qt. In any case, it is later checked that the selected
tools are the right ones and a warning is issued if there is a mismatch.